Yes. I'm sure your OB has been carefully watching the growth of your baby. Just because your stomach appears smaller, your baby is likely measuring appropriately on ultrasound. Everyone is different but you should discuss your concerns with your doctor.

Probably. You can't really tell by looking how big the baby is. The doctor can measure for the size of the uterus when you lay down. If problems are suspected an ultrasound can be done to find out in great detail!

Fetal growth. If the fetus is growing normally than "stomach size" doesn't matter. Fetal growth is assessed by uterine fundal heights measured by your doctor. A better way to assess fetal growth is a third trimester ultrasound and estimated fetal weight.